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Practical Process Control System Questions & Answers – 20

The compressor emergency shutdown system (ESD) has tripped the natural gas compressor off-line three times in the past 24 hours. Each time the operator goes to reset the compressor interlock, he notices the graphic display panel on the interlock system says “Separator boot high level” as the reason for the trip.

After this last trip, operations decides to keep the compressor shut down for a few hours until your arrival to diagnose the problem. Your first diagnostic test is to look at the indicated boot level in the sightglass (LG-93). There, you see a liquid level appears to be normal:

compressor emergency shutdown system

First, explain why this first diagnostic test was a good idea. Then, identify what would your next diagnostic test be.

Finally, comment on the decision by operations to leave the compressor shut down until your arrival. Do you think this was a good idea or a bad idea, from a diagnostic perspective? Why or why not?

Answer :

Given the fact that the ESD system keeps indicating a high boot level, you know that it “thinks” the liquid level inside the boot is higher than it should be. The next logical step is to determine whether or not a high liquid level condition does indeed exist. If so, the trip is legitimate and there may be a problem with the liquid level control system. If not, the LSHH-231 or its associated wiring may have a fault that sends a false trip alarm to the ESD system.

However, the decision to leave the compressor idle for a few hours until your arrival was not a good one for diagnosis. If indeed there is a problem with excessive liquid collecting in the boot, this would only be evident during running operation. With the compressor idle and no new gas entering the separator vessel, there will be no new liquid collecting in the boot, which will give the boot level control system ample time to empty that liquid down to a normal level and make it appear as though there is no level problem. In other words, leaving the compressor idle for a few hours “erases” the evidence, making it more difficult to troubleshoot.

Aside from re-starting the compressor and watching it run, you could perform a test on the liquid level control system by simulating a high-level condition inside the boot (e.g. applying pressure to one side of LT-92) and observing how fast or slow the actual liquid drains out (as indicated by LG-93). If there is a problem with the level control valve LV-92 or its associated components, you should be able to tell in the form of a long (slow) drain time. The fact that the blind flange at the bottom of the boot drain line says “Rod out” on the P&ID suggests this line is prone to plugging with debris, which could explain a slow-draining condition and consequently the frequent high-level trips.

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